Here is the fifth video in Engineer Guy Series #4. The element of the week is cesium, as in “cesium fountain atomic clock.” Watching it, my jaw was on the floor by 0:20, as Bill opens by showing off the Symmetricom CSAC, which is the world’s first fully functional chip-scale atomic clock. It’s about the size of a quarter. Bill then segues, thusly, into the general theory of atomic clocks: “I’ll start with Jell-O,” he says, producing a quivering green mound of it. And it just gets better from there, wrapping up with a clear, concise explanation of the Global Positioning System, and why it couldn’t work, at all, without the use of atomic clocks. [Thanks, Bill!]

I am descended from 5,000 generations of tool-using primates. Also, I went to college and stuff. I am a long-time contributor to MAKE magazine and makezine.com. My work has also appeared in ReadyMade, c’t – Magazin für Computertechnik, and The Wall Street Journal.

indeed such a wondrous fine video is worthy to post adjacent redundant links to it.

one miniscule quibble with the otherwise excellent video: two (3space) sphere surfaces intersect to produce an uncertainty of a (2 space) circle; a third sphere intersects with that circle at two points, how is that enough to establish a single point? …probably time constraints in the video prevent making that clear to simple folks such as me.

MrDan

Because of the 2 points, only one of them is on the surface of Earth. The other is in space somewhere.

The ISS can just ignore the point which doesn’t fit their normal altitude.

miroslava von schlochbaum

ok… all fair’nuff… but what happens if those two points happen to both be acceptable solutions (by being on the earth’s surface, or near the ISS’s ‘normal’ altitude)? (“that can’t happen.” ..ok, but why? “go visit a text book on spherical geometry and stop bothering us”)

ChrisW

True story about atomic clocks. The TV station I work at had a Rubidium atomic clock which we used to synchronize with the TV network. One day it failed and reverted to the quartz crystal oscillator. We didn’t notice any problem, but NASA called us to let us know it had failed. They had been using our broadcast signal as a frequency reference for some equipment. They gave us a Cesium clock to replace our failed Rubidium clock.

MrDan

3 satellites will get you 2 points, if you can use common sense (“I am stood on the Earth”), then you know which of those points is you. If not, you need a 4th satellite.

Now, the question is, how do the GPS satellites know where they are?

ChrisW

They are tracked by ground stations and
told where they are.

ChrisW

Good question. Usually, one of the points is not near the Earth’s surface. The other is your position.
His explanation was good but a little off. If only one satellite is received you know nothing about your position unless you have your own synchronized atomic clock. Distance is calculated using “pseudoranges”, and four satellites are required, I think.